Vacuum tubes may be the earliest electrical signal amplifiers. Vacuum tubes may be used to switch or to modulate electrical signals. They had been used in electronic devices such as televisions, radios, and loudspeakers, etc., for many decades. Nowadays, vacuum tubes are still used as audio power amplifiers in niche applications such as premier sound systems because of the low noise and high stability constants. Vacuum tubes may often be fabricated by mechanical machining processes and used as discrete components, and it may be not easy to integrate vacuum tubes with other devices. In contrast, semiconductor devices may be easily fabricated, and also have advantages such as low cost, low-power consumption, lightness, long lifetime, and ideal form factor for integrated circuits. Therefore, the vacuum tubes have been gradually substituted by the semiconductor devices.
In spite of that, vacuum tubes may still possess a plurality of advantages compared with semiconductor devices, such as high carrier mobility, and high output frequency, etc. Further, vacuum tubes may be more robust than semiconductor devices in extreme environments involving high temperature, and/or exposure to various radiations.
With the development of technologies, size of vacuum tubes has been reduced, and vacuum tubes have been integrated in existing integrated circuits (ICs) to enhance the performance of ICs.
FIG. 1 illustrates an existing vacuum tube. The vacuum tube includes: an emitter 11 on the surface of a substrate 10 at the bottom of the vacuum tube; a planar collector 20 at the top of the vacuum tube; and a vacuum chamber 40 between the substrate 10 and the collector 20, and vertical to the substrate 10 and the collector 20. The vacuum tube also includes a gate 30 between the substrate 10 and the collector 20. The gate 30 is parallel to the substrate 10, and surrounds the chamber 40. A partial of the gate is in the chamber 40. The gate 30 and the substrate 10 are isolated by a first isolation layer 31. The gate 30 and the collector 20 are isolated by a second isolation layer 32.
The emitter 11 may be a circular cone, and the collector 20 may be a planar electrode. Electrons emitting from the emitter 10 may reach the collector 20 through the vacuum chamber 40. Motions of the electrons in the vacuum chamber 40 may be controlled by an voltage applied on the gate 30.
However, the performance of the existing vacuum tube may be unstable, and the fabrication of the existing vacuum tube may be difficult. With the shrinkage of the device, the size of the vacuum chamber 40 may shrink correspondingly, and the difficulty for directly etching the vacuum chamber 40 may become more prominent. The disclosed methods and devices are directed to solve one or more problems set forth above and other problems.